Various processes are known to clean exhaust gases from combustion or burner installations; European Patent Application 0 118 017, Kroll et al, describes a gas cleaning apparatus to generate a spray, and in which a heat exchanger is provided. A cleaning liquid is circulated in a circulation loop to generate the spray. A neutralization substance, for example washing soda or sodium carbonate, is added to the liquid through a valve. Sulfur dioxide which is contained within the flue gases forms a sulfate in the liquid. The sulfate is a mud-like precipitate, which is removed from the bottom of a settling tank or vessel by suction, dried in a drying chamber, and removed from time to time. Turbulence of the precipitated material by a shaft penetrating into the bath is prevented by providing a shielding wall within the settling tank or vessel. This, however, does not prevent fine mud particles from being carried along in the circulation loop circulating the liquid; the particles which are carried along are tiny and represent, by weight, only a minor fraction of the overall circulated quantity. Any slightly larger solid particles are precipitated and permitted to settle out.
Encrustations result when alkaline earth compounds are used as neutralization substances. These encrustations interfere with the proper operation of the flue gas cleaning apparatus after relatively short time intervals.
The referenced article Catalog No. B 18 655 in the Swiss Federal Technological University, discusses the effect of pH values when cleaning flue gases, and points out that the pH values are of substantial importance. Highly alkaline solutions would be best suited to remove SO.sub.2 from flue gases. It has been found, however, that at a pH value of more than 8, CO.sub.2 which is present in flue gases in substantial excess is undesirably highly accepted by the neutralization substances, so that the quantity of neutralization substance required increases excessively. The article mentions that a suitable range for SO.sub.2 absorption is a pH value of from about 5 to 7.5. Direct absorption of SO.sub.2 with alkaline earth compounds Ca(OH).sub.2, CaCO.sub.3, Mg(OH).sub.2) has substantial problems, as referred to on page 16 of the literature reference. The reason appears to be that, almost exclusively, suspensions are involved, resulting in substantial danger of formation of encrustations. This is one of the worst problems of the first generation of processes in this field. The danger of encrustation is particularly high when using lime as a neutralization substance, as referred to on page 17 of the cited literature, so high that counter-flow absorption appears to be impossible. The separation degree is only up to about 90%. An advantage of using lime, however, is the low price of the alkaline earth absorbing material and that the resulting end products could be readily disposed of, for example in fills, or may even have sales value.
The Attisholz-Sulzer recycling process, described in the "Technical Review Sulzer", January 1987, page 22, article by Gage et al, describes a process in which a suspension of calcium carbonate and washing liquid is used. This suspension is pumped to the head of a column and flows in counter-flow direction with respect to the flue gases over a packing in the column. The calcium carbonate reacts with the sulfur dioxide and is converted to calcium sulfite and, partially, to water-soluble calcium bisulfite. Formation of sulfite in the absorption column results in increasing encrustation of the packing of the column. For a continuously operating system, two such columnar plants are necessary. This permits operating one column while flushing or rinsing or purging the other one. Any one column can operate for about 100 hours before requiring flushing or purging.